Integrated circuits are formed of a variety of different layers that are built up, one upon another, where each layer is patterned so as to construct structures that perform a variety of different functions. Typically, integrated circuits have been extremely customized. By this it is meant that a given integrated circuit design, even though it may use transistors, resistors, capacitors and other structures that are common with other designs, tends to have those structures disposed in unique locations and as parts of uniquely designed circuits. Thus, the mask sets for an integrated circuit design have typically been usable only for a single integrated circuit design, with different designs requiring entirely separate mask sets.
Masks have typically been fabricated according to one of two different designs. In a first mask design, the mask contains the image patterns required for a single given pattern layer of a single given integrated circuit design. Typically, the image patterns on the mask cover as much of the surface of the mask as is practical. The benefit of this mask design is that the image patterns on the mask tend to cover a large portion of, or even all of, the surface area of a substrate to be patterned with the mask. This is typically referred to as a wide field exposure mode, because the aperture of the exposure tool is opened wide to expose all of the images on the mask. Thus, this mask design provides for a relatively high throughput of substrates through the imaging process that employs such a mask. However, mask costs tend to be quite high when using this type of mask, because a separate mask is required for each masking layer of each integrated circuit design.
In the second mask design, the mask contains different blocks of image patterns required for several given pattern layers of a single given integrated circuit design. Typically, the blocks of image patterns on the mask are limited in size, so that more blocks can be fit onto the surface of the mask. The benefit of this mask design is that mask costs are reduced, because fewer masks are required to complete the integrated circuit design. This is because the image patterns for several different layers of the integrated circuit design can be placed onto a single mask. Thus, this mask design provides for reduced mask cost. This is typically referred to as a narrow field exposure mode, because the aperture of the exposure tool is restricted so as to only expose the desired portion of the images on the mask. However, throughput of the substrates through the imaging process tends to be quite low when such a mask is utilized, because the smaller block of image patterns for a given layer must be exposed and repeated multiple times across the surface of the substrate in order to completely image the substrate with the block of patterns.
What is needed, therefore, is a system whereby the strengths of various mask designs can be combined to both increase the efficiency of the exposure process and decrease the costs associated with masks.